The present invention mainly relates to a low-pass filter that is used in VHF, UHF, microwave and milliwave bands.
FIGS. 18A and 18B are schematic views illustrating a configuration of a conventional low-pass filter described in, for example, Japanese Patent Application Laid-open No. Hei 3-128501. In FIGS. 18A and 18B, reference numeral 1 denotes an external conductor formed in a housing shape of a rectangular parallelepiped; 2 denotes a dielectric substrate provided in such a manner that it partitions inside of the external conductor 1 into two at its center; and 3 denotes foil-like internal conductors formed by etching in a pattern zigzagged opposing both sides of the dielectric substrate 2, each of which is composed of a plurality of wide parts 3a and narrow parts 3b and 3c. 
Four wide parts 3a are disposed adjacent with each other and on a substantially straight line. Three narrow parts 3b are provided to electrically connect the wide parts 3a in series and are respectively bent at a right angle at two points. In addition, the narrow parts 3c are led out from the wide parts 3a at the both ends.
Reference numeral 4 denotes dielectric rods interposed between the narrow parts 3a on both sides of the dielectric substrate 2 and the internal surface of the external conductor 1. Reference numerals 5 and 6 denote coaxial input and output terminals provided in the external conductor 1, each central conductor of which is connected to the wide parts 3c. Reference numeral 7 denotes high impedance lines consisting of the narrow parts 3b and 3c and the external conductor 1. Reference numeral 8 denotes low impedance lines consisting of the wide parts 3a, the external conductor 1 and the dielectric rods 4.
Operations of the low-pass filter shown in FIGS. 18A and 18B will now be described with reference to its equivalent circuit diagram shown in FIG. 19. In FIG. 19, reference characters L1 to L3 denote inductors, which correspond to the high impedance line 7 and whose induction is determined according to line widths of the narrow parts 3b and 3c. Reference characters C1 and C2 denote capacitors, which correspond to the low impedance line 8 and whose capacitance is determined according to a line width of the wide parts 3a and a dielectric constant of the dielectric rods 4.
Here, the high impedance lines 7 and the low impedance lines 8 are required to perform pseudo-functions as an inductor and a capacitor of a lumped-constant circuit, respectively, and the respective axial lengths are set sufficiently smaller than a wave length of a pass-band frequency. In addition, reference characters Cp2 and Cp3 denote capacitors for giving an attenuation pole to a passing characteristic, which correspond to a combined capacity between adjacent low impedance lines 8 and whose capacitance is determined according to a distance between adjacent wide parts 3a. 
As described above, the conventional configuration shown in FIGS. 18A and 18B is represented by the equivalent circuit shown in FIG. 19, and therefore has a function as a low-pass filter.
Moreover, an inductor Li (i=1, 2, 3, . . . ) and a capacitor Cpi form a parallel resonance circuit with a resonance frequency of f0       f    0    =            1      2        ⁢                            L          i                ⁢                  C          pi                    
Thus, if values of Li and Cpi are set such that this parallel resonance circuit operates to have necessary inductance as a whole at a frequency of a pass-band f0 a filter and generates parallel resonance at a frequency higher than the pass-band, that is, a stopping band frequency f0, the passing characteristic of this filer becomes a low-pass characteristic having an attenuation pole in the resonance frequency f0 as shown in FIG. 20. Therefore, a low-pass filter having a steep out-of band attenuation characteristic is obtained by placing this resonance frequency f0 at an appropriate position of the stopping band.
Since the conventional low-pass filter is composed as described above, a length of a section combining the adjacent low impedance lines 8 is relatively short and, in particular, if a line is formed with a uniform medium such as a triplet line, the coupling of the adjacent low impedance lines 8 cannot always be sufficient. Thus, there is a problem in that a large value cannot be obtained as capacitance of the capacitor Cpi and it is difficult to set the attenuation pole frequency f0 as low as in the vicinity of the pass-band.
The present invention has been devised to solve the above and other problems, and it is an object of the present invention to provide a low-pass filter that can set an attenuation pole in the vicinity of a pass-band and has a steep out-of band attenuation characteristic even if the low-pass filter has a simple configuration of a plane circuit consisting of a line such as a triplet line and a microstrip line.
According to the present invention, there is provided a low-pass filter comprising: combined lines formed of three or more top end open stubs, which are set to have a large electric length in a range in which a length is shorter than xc2xc of a wavelength of a pass frequency and disposed substantially in parallel in such a manner that an open end of each of the three or more top end open stubs faces an identical direction; and a high impedance line connected to at least one part among parts between neighboring ends that are on the opposite side of the open ends of the top end open stubs and having a length shorter than the wavelength of the pass frequency.
Also, the high impedance line is a first high impedance line, and the low-pass filter further comprises, in addition to the first high impedance line, at least one second high impedance line that is connected at one end to ends on the opposite side of open ends of top end open stubs among the both ends of the three or more top end open stubs and has a length shorter than the wavelength of the pass frequency.
Further, the low-pass filter further comprising a low impedance line that is connected to at least one the other end of the second high impedance line at one end and has a length shorter than the wavelength of the pass frequency.
Still further, a multi-stage filter is formed by cascading low-pass filters in a plurality of stages via a high impedance line.
Yet still further, the low-pass filter is formed of a triplet line.
Further, the low-pass filter is formed of a micro-strip line.
Furthermore, the low-pass filter is formed of a coplanar line.
According to another aspect of the present invention, there is provided a low-pass filter comprising: combined lines formed of three or more top end short-circuit stubs, which are set to have a large electric length in a range in which a length is longer than xc2xc and shorter than xc2xd of a wavelength of a pass frequency, and disposed substantially in parallel in such a manner that each of short-circuit ends of the three or more top end short-circuit stubs faces an identical direction; and a high impedance line connected to at least one part between ends among parts between ends that are on the opposite side of the short-circuit ends of the top end short-circuit stubs and adjacent with each other and having a length shorter than the wavelength of the pass frequency.
Also, the low-pass filter is formed of a triplet line.
Further, the low-pass filter is formed of a micro-strip line.
Furthermore, the low-pass filter is formed of a coplanar line.
Still further, the low-pass filter has a first conductor layer, a second conductor layer and a third conductor layer, which are disposed with the second conductor layer being sandwiched between the first and the third layers, and a ground conductor formed on external surfaces of the first and the third conductor layers, and is composed of a multi-layer high frequency circuit in which a central conductor is formed on the front and the back of the second conductor layer, and has a strip conductor forming a central conductor of a top end open stub and a strip conductor forming a central conductor of a high impedance line that are formed separately on the front side and the back side of the second conductor layer.
Yet still further, the low-pass filter has a first conductor layer, a second conductor layer and a third conductor layer, which are disposed with the second conductor layer being sandwiched between the first and the third layers, and a ground conductor formed on external surfaces of the first and the third conductor layers, and is composed of a multi-layer high frequency circuit in which a central conductor is formed in the front and the back of the second conductor layer, and has a strip conductor forming a central conductor of a top end short-circuit stub and a strip conductor forming a central conductor of a high impedance line that are formed separately on the front side and the back side of the second conductor layer.
Furthermore, the low-pass filter has a first conductor layer, a second conductor layer and a third conductor layer, which are disposed with the second conductor layer being sandwiched between the first and the third layers, and a ground conductor formed on external surfaces of the first and the third conductor layers, and is composed of a multi-layer high frequency circuit in which a central conductor is formed in the front and the back of the second conductor layer, has each strip conductor forming a central conductor of three or more top end open stubs forming a combined line, to which a high impedance line that is shorter than the wavelength of the pass frequency is connected between ends on the opposite side of the open ends of the top end open stubs adjacent with each other, provided on the front and the back of the second dielectric layer with sides opposing each other, and each strip conductor forming a central conductor of the high impedance line is connected to each strip conductor of the top end open stubs to be provided on the front and the back of the second dielectric conductor layer and connected via a through-hole in the middle.
Finally, the combined lines are a pair of combined lines disposed substantially in parallel in such a manner that each open end of the three or more top end open stubs faces an identical direction, and are connected in parallel such that the ends on the opposite side of open ends of the top end open stubs in each of the pair of combined lines are opposed to each other to be connected, and the low-pass filter is provided with a high impedance line which is connected to at least one part among parts between neighboring ends on the opposite side of the open ends of the top end open stubs and is shorter than a wavelength of a pass frequency, and has a first conductor layer, a second conductor layer and a third conductor layer disposed with the second conductor layer being sandwiched between the first and the third conductor layers and ground conductors formed on the external surfaces of the first and the third conductor layers, and is composed of a multi-layer high frequency circuit in which a central conductor is formed on the front and the back side of the second dielectric layer, and has each strip conductor forming a central conductor of the top end open stubs formed on one side of the second dielectric conductor layer, and a strip conductor forming a central conductor of the high impedance line formed on the other side of the second dielectric layer, and in which the connection between ends on the opposite side of the open ends of the top end open stubs and the high impedance line is made by the connection via a through-hole of a strip conductor forming a central conductor formed on the front and the back side of the second dielectric layer.